The easiest way is to turn off autocorrect!

I leave spellchecking active, which just gives an indication with a squiggly underline when it does not find the word in its dictionaries and affix rules. You can then decide what to do: look at the suggestions, select a suggestion for replacement, add the word to the personal dictionary, or simply ignore the warning. Especially with slang or local dialects you often just have to ignore the warnings, since the spell checker will be based on either Websters dictionary, Oxford dictionary or “The Queen's English” (I doubt that M$ has yet managed to replace that with “The King's English”). (Pun intended and tongue firmly in cheek.)